This invention relates to a magnetic recording medium, and more specifically to a high-density magnetic recording medium containing .alpha.-Al2O3 particles in the magnetic layer thereof.
In the manufacture of a magnetic recording medium, for example, in the form of audio or video tapes or data memory media, it is commonly in practice to add a nonmagnetic inorganic abrasive material such as Al2O3, Cr2O3, Cr2O3, TiO2, SiO2, SiC, .alpha.-Fe2O3, or ZnO2 to the magnetic layer to protect the latter against damage due to contact with the magnetic head.
Proper choice must be made of the abrasives, particle size, and quantity of the inorganic material powder to be added; otherwise, the particles accelerate the wear of the magnetic head or deteriorate the electromagnetic characteristics of the medium. One approach to minimize these drawbacks has been to reinforce the magnetic layer by a small amount of addition of an abrasive powder having a mean particle diameter of about 0.5 .mu.m. Tapes using metallic magnetic particles have a disadvantage of being easily damaged by contact with the head since the metallic powder is lower in hardness than the iron oxide powder. A general solution employed to this problem has been mixing the metallic magnetic powder with a nonmagnetic powder of greater hardness, however it requires by far the larger amount of the reinforcing material than the iron oxide powder does.
The present inventors have made their studies in search of a magnetic recording medium excellent in electromagnetic characteristics and a durability with a minimum of head clogging through suitable choice of the abrasive and its particle size of an inorganic material as a coated film reinforcing material.
In recent years, magnetic powders of finer particles, more highly dispersible binders, and smoother tape surfaces have come into extensive usage to meet the requirements for higher performance video tapes and tapes for digital equipment. In these applications the reinforcement of the coated film by the addition of the high-hardness nonmagnetic powder tends to affect adversely the durability of the magnetic coated film or the reduction of deposition on the head. In view of these, there has been a demand for some method of achieving improvements in the durability of the coated film and also in the head-cleaning effect.
The commonly used inorganic material particles, when incorporated in such high-performance tapes, would pose problems. For example, the addition can deteriorate the electromagnetic characteristics, damage the tape during runnings due to an insufficient reinforcing effect, or lead to head clogging heavy enough, for example, to mar the picture quality of the video tape. The present inventors thought that the problems involved in the enhancement of performance could be settled by the proper selection of the particle size as well as the kind of an inorganic material as a coated film reinforcing material.
This application previously found (Japanese Patent Application No. 92405/1986) that the addition of .alpha.-Al2O3 particles having a mean particle diameter in the range of 0.1 to 0.3 .mu.m to the magnetic layer of a magneetic recording medium using fine ferromagnetic powder with a specifric surface area of 35 m.sup.2 /g or more as determined by the BET method would satisfy the otherwise incompatible requirements for the reduction of damage and wear of the magnetic head and for the reduced wear of the magnetic tape. Nevertheless, the disclosure has still been unsatisfactory in respect of the wear of the head.
The present invention therefore has for its object of the provision for a magnetic recording medium which causes less clogging, wear, or damage of the head and less wear of the magnetic layer of the medium than heretofore.